Method and apparatus for focus and tracking in an optical disk system

ABSTRACT

Method and apparatus useful in monitoring and adjusting focus and tracking of a read/write beam in a data disk-storage system are provided. In one embodiment, the reflected beam is split into a plus first order, astigmatically focused first beam and a minus first order astigmatically focused second beam. The two beams produce images which change in a mirror symmetric fashion in response to changes in the focus or tracking but in a parallel fashion in response to changes in the optical axis. Signals from detectors of the two beams are combined to negate the influence of optical axis movement or other optical faults. In another embodiment, a beam reflected from the disk is split into a first beam converging on an image plane and a second beam defocused at the image plane. The second beam has a relatively large diameter at the image plane and is relatively insensitive to optical axis deviations. Phase relationships of the second beam are used to determine if the beam impinging of the disk is focused. By combining signals from the first and second beams, the resultant signal is insensitive to optical axis errors and indicates the magnitude and direction of focus correction needed. The second beam is also used in monitoring or measuring tracking of the read/write beam. In both embodiments, the beam splitting is preferably produced by a zone plate.

This is a division of application Ser. No. 07/079,653 filed July 29,1987, now U.S. Pat. No. 4,879,706.

FIELD OF THE INVENTION

The present invention relates to information storage on rotatable disksand particularly to apparatus and method for monitoring and controllingtracking and focusing of a light beam with respect to an opticallyreadable disk.

BACKGROUND OF THE INVENTION

Disk-type data storage systems, and particularly optically readable disksystems, require proper placement of a focused beam of light, such as alaser beam, with respect to the disk. Placement in the axial direction,i.e. towards or away from the disk surface, involving focusing of thelight beam and placement in the radial direction, referred to astracking, are needed to assure that a light beam of the required sizeand intensity impinges on the disk at the desired location.

Methods for monitoring and controlling tracking and focusing of the beamtypically involve detection of characteristics of at least a portion ofthe beam which is reflected from the disk surface. One such system isdescribed in U.S. Pat. No. 4,446,546 issued May 1, 1984 to Miller. Thissystem uses a quadrature detector which produces an "S" curve for use infocus control.

A persistent problem with such systems is that commonly-used detectingdevices are unable to distinguish between characteristics which resultfrom defocusing or track crossing and characteristics which result fromother changes in the optical system such as movement or misalignment ofthe optical axis of the reflected beam or misalignment of opticalcomponents of the system. Because of this inability to distinguish, amisalignment of some components of the optical system can result in thedetector system producing a focus control signal or a tracking controlsignal which is erroneous, i.e. which causes the focused point of thelight beam to be positioned away from the desired position.

A number of methods have been devised to overcome or compensate for thelack of signal discrimination. If the optical detectors are relativelysmall compared to the size of the beam being detected, movement of thebeam axis with respect to the detectors may leave the detectorssubstantially within the beam and thus substantially unaffected by suchmovement. However, for many focus correction systems, the beam beingdetected must be focused on the detector. Producing a focused spot whichis sufficiently large to avoid the effects of axis movement during theanticipated life of the device places constraints on other components,such as lens apertures and bit density, which make this solutionimpractical and expensive to implement.

The tracking system can be separated from the focus system by, forexample, a beam splitting arrangement intended to provide a relativelyerror-free tracking system. However by splitting the optical path intotwo optical paths which pass through different optical elements, eventssuch as optical axis movement in one path may not occur in the otherpath and thus signals related to one optical path may not be useful incorrecting errors in the other optical path. Furthermore, thisarrangement is beneficial only to the tracking system and does not solveproblems associated with the focus system.

Conversely systems such as that disclosed in U.S. Pat. No. 4,123,652,issued Oct. 31, 1978 to Bouwhuis, include a focusing system which isintended to overcome certain optical faults. However, Bouwhuis does notdisclose overcoming optical faults in the tracking system. Furthermore,Bouwhuis requires splitting of the optical path thus creating thepossibility for independent optical faults in the two optical paths.

Accordingly, there is a need for a system for tracking and focusing anoptical beam which is insensitive to optical faults such as beam axismovement or component misalignment and which does not involve subjectingsplit beams to independent optical faults.

SUMMARY OF THE INVENTION

The present invention involves apparatus and methods useful inmaintaining focus of a light beam on a data disk and maintainingtracking of a light beam on a data disk.

In a first embodiment a light beam is reflected from the data disk andthe reflected beam is subjected to the influence of an optical elementwhich simultaneously produces two beams. The two beams are preferably aplus first order beam converging to form a first image on an image planeand a minus first order beam converging to form a second image,preferably on the same image plane. The two images are thus focusedpreferably on a single image plane, spaced from each other. The firstimage changes in a mirror symmetric fashion with respect to changes inthe second image when there is a change in the focusing of theread/write beam on the disk. The first image also changes in a mirrorsymmetric fashion with respect to changes in the second image when thereis a change in tracking of the read/write beam with respect to the disk,for example, when there is detection of a track crossing, a bit edge, ortrack servo bit. However, the first image changes in a parallel fashionwith respect to changes in the second beam when there is a change in theoptical axis of the reflected beam. Because the undesired changesresulting from optical faults, for example, optical axis movement,create parallel results in the two images, the two images can becombined in a fashion to nullify such unwanted effects, and thereby toproduce a combined signal which reflects substantially only changes intracking or focusing and which is substantially unresponsive to changesin the optical axis of the reflected beam.

According to this first embodiment, the optical element can also producea zero order beam which preferably converges to form a third image onthe image plane which is useful for detecting modulation of theread/write beam by the disk. The optical element has an astigmatizinginfluence on the plus first order and minus first order beams, useful inconnection with the focusing system. The optical element can beconfigured to produce astigmatism of the first beam which is mirrorsymmetrically oriented with respect to the astigmatism of the secondbeam. In this manner, a single element can produce a desiredrelationship of two astigmatic focus lines, for example, an orthogonalrelationship, without the necessity for separately aligning twodifferent optical elements such as two lenses.

In a second embodiment, the light beam is reflected from the data diskand the reflected beam is subjected to the influence of an opticalelement which again simultaneously produces two beams. The first beam isastigmatically focused by the same optical element to an image in animage plane. The second beam is affected by the same optical element soas to be defocused in the image plane. The first and second beams arepreferably coaxial. Two different detecting means are used for the firstand second beams. The first detector means detects the focused imagefrom the first beam. The first detector is preferably a quadraturedetector which produces an "S" curve from the astigmatically focusedfirst beam. The second detector includes substantial portions outsidethe image from the first beam and is responsive to the defocused secondbeam. The second detector, being substantially smaller than the size ofthe defocused second beam, is substantially unaffected by optical faultssuch as movement of the optical axis of the defocused beam. The phaserelationship detected by the second detector is used to modify the "S"curve produced by the first detector. The resultant signal is related tothe degree of focus of the beam on the disk and is substantiallyunaffected by movement of the optical axis.

The second detector is also used in monitoring and controlling trackingof the beam with respect to the disk. As noted, the tracking signal isrelatively unaffected by optical faults, such as movement of the opticalaxis, because, as noted, the second detector is relatively smallcompared to the size of the defocused second beam at the image plane.

The single optical element which accomplishes creation of two beams inboth the first and second embodiments is preferably a zone plate. Byusing a single optical element, as opposed to a combination of elementssuch as a number of lenses material expense and production andmaintenance costs are reduced. Furthermore, even when a number ofoptical components are combined to produce the desired effect, the firstbeam and second beam both pass through the same optical elements so thatconditions causing an optical fault in one of these beams also causes anoptical fault in the other. For this reason, a signal from a detector ofone beam can be combined with a signal from the detector of the otherbeam to produce a final signal which is relatively free from theinfluence of optical faults.

According to both embodiments of the invention, the detector apparatuscan be provided in a single compact device because the detectors are allpreferably located in a single image plane and are located closetogether. By providing a single compact device, material andconstruction costs are minimized and maintenance requirements arereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a system for focusing a coherentlight beam onto a disk track and for generating focus and trackingsignals;

FIG. 1A is a front view of a quadrant detector taken along Line 1A--1Aof FIG. 1;

FIGS. 2A-2C depict the quadrant detector of FIG. 1A displaying threedifferent focus conditions;

FIG. 3 is a schematic depiction of another focus and tracking signalsensing system;

FIG. 3A depicts the detector taken along Line 3A--3A of FIG. 3;

FIG. 4 is a schematic representation of a detector for sensing focus andtracking error according to a first embodiment of the invention;

FIG. 5 is a front view of a detector taken along Line 5--5 of FIG. 4;

FIGS. 6A, B, C and D depict the quadrant detector of FIG. 4 displayingthree different focus conditions and an axis movement;

FIGS. 7A and B depict the diffraction pattern of the refracted beambefore and after rotation by the optical element;

FIG. 8 is a schematic representation of a monitoring apparatus forimaging and focus error signal generation according to the secondembodiment of the invention;

FIG. 8A is a schematic front view of an optical element of FIG. 8 takenalong Line 8A--8A of FIG. 8;

FIG. 8B is a front view of the detector taken along Line 8B--8B of FIG.8;

FIG. 9 is a phase diagram showing the phase relationship from twodetectors in a focusing and tracking detection system according to thefirst embodiment of the invention;

FIG. 10 is a schematic depiction of a detector configuration for sensingfocus and tracking error signals which is an alternative configurationof the second embodiment of the invention;

FIG. 10A is a front view of a detector taken along Line 10A--10A of FIG.7;

FIG. 11 is a schematic representation of a point light source and itswavefront at a plane P; and

FIG. 12 is a diagramatic depiction of various stages in the constructionof an elliptical zone plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to tracking and focusing a light beam inconnection with storage and retrieval of data in a disk data-storagesystem. Referring to FIGS. 1 and lA, depicted therein is a prior artoptical disk drive system in which a light beam is produced from acoherent light source 10, such as a semi-conductor laser, preferablywith polarization parallel to the plane of incidence of the polarizingbeam splitter 14. The beam is directed to a mirror 12, to a polarizingbeam splitter 14, and through a quarter wave plate 16. The quarter waveplate 16 changes the linear polarized light which is transmitted by thepolarization beam splitter into circular polarized light, as known. Thelaser beam is then focused onto the surface of the disk 20 by theobjective lens 18 to a spot having a diameter of, e.g., about 1micrometer or less on the surface of a recording medium 20, such as aconventional rotating disk. To achieve such a spot, the objective lens18 with a large numerical aperture of, e.g., about 0.52 is used. Withlarge numerical aperture optics, the depth of focus of such an objectivelens is small. Therefore, it is desirable that the objective lens bemaintained at a preferred distance from the medium surface 20 so thatthe light beam remains sharply focused on the plane of the informationtrack on the disk 20. In an optical disk drive, the separation of thesurface of the medium track and the objective lens may tend to vary dueto, e.g., the vertical run out of the disk. This requires that someprovision be made to detect these variations and to correct the focusingerror in response thereto.

The disk medium 20 typically has a reflectance of 0.4. Reflected lightfrom the disk 20 passes back through the objective lens 18, the quarterwave plate 16, off the polarizing beam splitter 14, through anastigmatic lens such as a toric lens 22, and is focused on a quaddetector 24.

The toric lens 22 is fabricated by polishing two curvatures on the sameglass piece. This type of lens suffers from spherical abberation andrequires accurate centering in order to produce a uniform and wellshaped beam for detection. Another method for producing such a lens withtwo foci is to emboss a thin layer of material with a cylindricalcurvature on it. Such a lens is sometimes referred to as a bifocal lensand is of higher quality than a toric lens but at higher cost.

The light beam which is focused on the surface of the disk 20 shouldalways remain centered on an information track as defined bytrack-defining systems such as by pregrooved portions to be read (orwritten), bit edges or tracking servo bits. Deviations from suchcentered position can produce a reduction of the modulation depth of theread-out signal and/or crosstalk between adjacent track portions.Centering of the focused beam on the desired track means that the centerof the focused beam should coincide with the center of the track. In thecase of an off-centered beam, the centers will not coincide. Therefore,the optical drive should include means for detecting the magnitude andthe direction of a centering error of the focused beam relative to aninformation track to be accessed so that the position of the spot can becorrected.

As in a typical prior art optical drive apparatus, such as the ModelLD1200 optical drive manufactured by Laser Magnetic StorageInternational Company of Colorado Springs, Colo., the tracking errorsare obtained by combining detector signals generated by electricaldetector zones, e.g., by combining in the fashion

    (a+d)-(b+c)                                                (1)

with reference to the quad detector 24 depicted in FIG. 1A. If the spoton the disk moves across, e.g., pregrooved data tracks the intensity ofthe light on the quad detector 24 will fluctuate, producing afluctuation in the signal from the detector. The fluctuation of thiselectrical detector signal is caused by the interference of thediffracted beams produced by the track-defining system.

The focus error signal is obtained by combining detector signalsgenerated by electrical detector zones, e.g., by combining in thefashion

    (a+c)-(b+d)                                                (2)

with reference to the quad detector 24 depicted in FIG. 1A.

The focus error signal is generated using the lens 22 which is usually aconventional toric lens as discussed above. Depending on the conditionof the axial position of the focused spot with respect to the surface ofthe disk 20 the light pattern on the quadrant detector 24 will have oneof the shapes illustrated in FIGS. 2A through 2C. When the spot isproperly focused, the shape of the light is approximately symmetrical,as depicted in FIG. 2B.

The spot on the quad detector 24 as shown in FIG. 2B has equal intensityon all four quadrants of the quad detector 24. On the other hand, whenthe laser beam is not in proper focus on the surface of the disk 20, thespot on the quad detector 24 will be astigmatic and show preference todiagonally opposed quadrant portions of the quad detector 24, asdepicted in FIGS. 2A and 2B. The electrical signal defined by Equation 2taken from the quad detector 24 will be reduced (compared to thein-focus signal) when the focus is in error in a positive sense (asdepicted in FIG. 2A) with the magnitude of the reduction being relatedto the magnitude of the focus error. The electrical signal obtainedaccording to Equation 2 will be increased (compared to the in-focussignal) when the focus is in error in a negative sense (as depicted inFIG. 2C) with a magnitude related to the magnitude of the focus error.In this way, the error signal can provide both the direction and themagnitude of the focus errors.

Both the tracking and focus systems described suffer from a number ofconstraints and shortcomings. In the particular configuration of errorsignal generation described above, the tracking error signal inaccordance with Equation 1 can only be accomplished when the spot isclose to an optimum focused condition. Moreover, the spot as shown inFIG. 2B is rather small. In the Model LD1200 optical drive mentionedabove, the spot diameter is about 250 micrometers. As a result, when thedrive encounters different thermal environments, the thermal expansionof the mechanical parts coupling with stresses built into some of theadjustments parts causes the beam on the quad detector 24 to move fromits pre-aligned position This beam motion produces an electrical offsetsignal which reduces the accuracy in monitoring the tracking on the disk20. For example, if the axis of an accurately focused beam shifts so asto move the spot shown in FIG. 2B towards quadrant b, the axis shiftwill cause the tracking error signal defined by Equation 1 to decrease,causing a change in tracking of the beam, even when the beam is properlycentered.

Such beam motion also affects accuracy of the focusing system. Thedescribed axis shift will cause the quantity defined by Equation 2 todecrease and the resulting focus error signal will mimic the signalproduced in an out-of-focus situation such as depicted in FIG. 2A. Thus,such an axis shift will cause the focus system to defocus a beam whichis, in fact, accurately focused.

FIG. 3 depicts another prior art optical drive system in which the laser10 directs a beam of coherent light through the polarizing beam splitter14 the quarter wave plate 16 the objective lens 18 and onto the disk 20.The reflected beam passes back through the objective lens 18, throughthe quarter wave plate 16 and out of the polarizing beam splitter 14. Asecond beam splitter 26 divides the return beam into two beams. Thetracking signal is sensed by a split detector 28, depicted in FIG. 3A.The focus error is sensed by passing the focus beam through a lens 22 tothe quadrant detector 24. In this approach, the tracking signal detectedon the split detector 28 becomes insensitive to the movements of, e.g.,the mechanical mounts in the system. However, the focus error detectedon the quad detector 24 is still influenced by beam tilt or other motionof the beam caused, e.g., by stresses built into the supportingmechanical mounts. Moreover, because the focus beam passes through thelens 22 while the tracking beam does not, no signal detected by detector28 could be used to discriminate optical faults which occur only in thefocus beam path, such as those that might be caused by misalignment ofthe lens 22.

According to a first embodiment of the invention, an optical elementwhich produces two focused beams is used. As depicted in FIG. 4 anoptical element 60 comprising for example a lens 62 and a zone plate 64are placed in the path of at least a portion of the beam reflected fromthe disk 66. The characteristics and method of making a zone plateusable in this embodiment are discussed below. Briefly, a preferred zoneplate for this embodiment would have raised or opaque regions in theshape of segments of ellipses. The effect of the optical element 60 isto produce two converging beams, a minus first order beam 68 and a plusfirst order beam 70. Each of these beams converges to an image planewhich is preferably the same plane for both beams 72.

The optical element 60 provides the two beams 68 70 with thecharacteristic that the shape and diffraction pattern of the beams atthe image plane 72 change in a mirror symmetric fashion when there is achange in the focus or tracking of the read/write beam but change in aparallel fashion when there is a change in the optical axis of thereflected beam 66. Further, the optical element 60 imparts astigmatismto the two beams.

The first and second beams 68, 70 are detected using two quadraturedetectors 74, 76 depicted in FIG. 5. The first quadrature detector 74 ispositioned to detect the first beam 68 and the second quadraturedetector 76 is positioned to detect the second beam 70. The quadraturedetectors 74, 76 are located substantially at the image plane 72 of thetwo beams 68, 70.

The optical element 60 can also be configured to provide a third or zeroorder beam 78 which converges to an image plane preferably coplaner withthe image plane 72 of the first and second beams 68, 70. In oneembodiment of the invention, the diameter of the image at the imageplane 72 of the zeroth order light beam 78 is about 10 microns A thirddetector 80 is positioned to detect the third beam 78 to provide asignal related to modulation of the read/write beam by the data disk.

FIGS. 6A, 6B, 6C, and 6D schematically depict the pattern of lightfalling on the detectors 74, 76 80 in the embodiment depicted in FIG. 4for different focus and optical axis conditions. FIGS. 6A, B and Cdepict situations in which the optical axis has not shifted, i.e., inwhich the first and second beams 68, 70 are centered on the centers ofthe quadrature detectors 74, 76, respectively. FIG. 6A depicts the lightpattern falling on the detectors 74, 76, 80 when the read/write beam isin focus. FIG. 6B depicts the light pattern falling on the detectors 74,76, 80 when the read/write beam is out-of-focus with respect to the datadisk in the positive sense. FIG. 6C depicts the light pattern falling ondetectors 74, 76, 80 when the read/write beam is out-of-focus withrespect to the data disk in the negative sense. FIG. 6C depicts thepattern falling on the detectors 74, 76, 80 when the read/write beam isin focus with respect to the data disk but the axis of the reflectedbeam 66 has shifted from the preferred or centered position.

According to this embodiment of the invention, the focus error signal isobtained by combining the outputs from the quadrature detectors k, l, m,n, p, q, r and s in the following manner:

    [(k+m)-(1+n)]+[(q+s)-(p+r)].                               (3)

The tracking error signal, according to this invention, is obtained bycombining the signals from the quadrature detectors in the followingfashion:

    [(k+n)-(1+m)]+[(q+r)-(p+s)].                               (4)

By combining the signals from the two quadrature detectors in thismanner, both the focus error signal and the tracking error signal aresensitive to the focus and tracking conditions of the read/write beamwith respect to the data disk, but are each relatively insensitive tooptical faults such as changes in the optical axis of the reflected beam66. As with the first embodiment, the ability to combine signals fromtwo beams to produce a signal which is insensitive to optical faults isrelated to the fact that the two beams follow non-independent paths,i.e. each beam passes through the same optical elements.

A system generating non-independent beams is further useful in that whenthe astigmatism of the respective focus beams 68, 70 is produced by asingle element 64, the astigmatic focus lines will be produced in apredetermined angular relatiOnship, preferably orthogonal. Thus, thereis no need to independently adjust two different optical elements suchas cylindrical or toric lens to assure orthogonality of the astigmaticfocus lines of the two beams 68, 70.

FIG. 7A depicts the position of the top and bottom diffraction patterns81 (denoted T and B in FIG. 7A) at a point along the reflected beam 66and FIG. 7B depicts the position of the diffraction patterns 82, 84 atthe image plane 72. As can be seen from FIG. 7, the optical element 60results in a ninety degree rotation of the diffraction patterns withrespect to each other. The rotations imposed on the diffraction patternsin the first and second beams 68, 70 are mirror symmetric in the sensethat when the diffraction pattern 82 of the first beam 68 is rotated 45degrees in a counterclockwise direction, the diffraction pattern of thesecond beam 70 will be rotated 45 degrees in a clockwise direction.Because the diffraction patterns are rotated in opposite directions bythe optical element 60, changes in the diffraction patterns which resultfrom tracking events such as track crossing, bit edge detection or servobit detection will be mirror symmetric at the two detectors 74, 76,while changes in the diffraction patterns which result from opticalfaults, such as movement of the optical axis of the reflected beam 66,will cause parallel movement of the diffraction patterns at the twodetectors 74, 76. In this way, the tracking error signal obtained inaccordance with Equation 4 will be relatively insensitive to opticalfaults.

FIG. 8 illustrates a second embodiment of the new focusing error andtracking error generation apparatus and method of the present invention.Rather than one optical element that produces two focused beams, thisdetection system includes an optical element for splitting the reflectedbeam 129 into two beams, one of which is astigmatically focused on theplane of the detector 132, the other being defocused at the plane of thedetector 132.

The optical element 130 can comprise any of a number of componentsincluding one or more lenses and one or more diffraction gratings. Apreferred component for the optical element is a zone plate element 130.The pattern of this zone plate element 130 can be produced as describedmore fully below. Briefly, a pattern for the plate is producedoriginally by a computer on a graphic device such as a pen plotter,photoplotter, CRT display or electron beam graphic output device. Apreferred pattern for the second embodiment includes elliptically-shapedregions. This computer output is converted into a phase relief typeoptical element by methods to be discussed below. FIG. 8A is an opaquerepresentation of the face of the optical zone plate 130 (not to scale)and is shown here to provide a conceptual visualization thereof. Aspointed out hereinbelow, the optical zone plate 130 need not have opaqueportions and, in general, better results in regard to light intensityare achieved with a transparent zone plate having a raised or loweredelliptical pattern.

FIG. 8B depicts the face of a modified detector plate 132 utilized inthe optical system represented in FIG. 8. One of the beams generated bythe optical element 130 is a converging beam similar to the astigmaticbeam from a toric lens, and is designated in FIG. 8 as first order. Thesecond beam therefrom is a defocused, nearly collimated beam with alarge beam diameter and is designated in FIG. 8 as zeroth order. A thirdbeam may also be produced by the optical element 130, being a divergingbeam, and is designated in FIG. 8 as minus first order. The convergentbeam and the collimated beam are sensed by the detector 132. Thedivergent beam in the embodiment depicted in FIG. 8 does not serve anyfunction, and thus represents some loss of total useful intensity in thedetected beams.

As in a typical optical drive, the beam diameter of the convergent beamat the quad detector, in one embodiment of the invention, is about 0.25mm. The diameter of the collimated beam is about 4 mm, and the diameterof a divergent beam at the detector plane is about 8 mm. Since thediameter of the divergent beam is large in comparison to the other twobeams, its contribution to the detected signals is small. FIG. 8Bschematically illustrates the three beams (plus first, zeroth, minusfirst) as these appear on the plane of the detector 132 (not to scale).In a tracking system which uses pregrooves, the inner two beams producea half moon shaped pattern which relates to the interferences resultingfrom the pregrooved structure on the recording medium of the disk 20.The detector depicted in FIG. 8B can also be used with tracking systemsnot employing pregrooved structures, such as servo bit tracking systems.

According to one embodiment of the invention, the tracking error signalis generated by the electrical detection zones by combining the signalsfrom the detection zones denoted e, f, g and h in FIG. 8B in thefollowing fashion:

    (e+f)-(g+h).                                               (5)

Because the collimated beam is large in comparison to the magnitude ofdeviations of the beam axis produced, e.g., under thermal stress, orother optical faults, the tracking signal thus generated is stable andnot sensitive to optical faults.

The small quadrant detector 133 in the center of the detector 132 isused to produce the astigmatic focus electrical error signal or "S"curve in the same manner as that using the signal from a conventionaltoric lens. However, as noted above, such error signal is sensitive tooptical faults. The present invention, therefore, includes using asignal from the defocused beam detectors e, f, g and h which arerelatively insensitive to optical faults, in correcting or compensatingfor the quadrature detector signals to produce a final signal which isrelated to the focus, but which is relatively insensitive to opticalfaults. The usefulness of this aspect of the invention is related to thefact that both the defocused tracking beam and the focused focusing beampass through the same optical components. Because the focused anddefocused beams pass through the same optical components, both beamswill be exposed to the same optical faults, enabling a signal from onebeam to be used in compensating for the effects of optical faults on theother beam.

In the present invention, the phase relationship between the signal fromthe detector zones e and f of the detector 132, or similar phaserelationship between the signal from the detector zones g and h, canindicate the true focus condition of the spot on the medium. Althoughthe following discussion is in terms of using a signal from either pairof detectors (i.e. e and f or g and h), preferably signals from bothpairs of detectors are additively combined resulting in a stronger totalsignal and, thus, a better signal-to-noise ratio. The difference signalof the two detectors indicates the sign of the focus error, while thesum of the two detectors can be used as a phase reference signal.Multiplication of both signals as in the form:

    (e-f)·(e+f)                                       (6)

yields a focus error signal, as described in Braat, et al., "PositionSensing in Video Disk Readout", Applied Optics, Volume 17, pp.2013-2021.

An example of the phase relationship of these signais is illustrated inFIG. 9, where the two traces represent idealized signals from detectorse and f or from detectors g and h. The phase, in radians, is the lag ofone signal, with respect to the other, divided by the common period ofthe signals. The focus error signal as described in the Braat, et al.Applied Optics article, however, has a disadvantage in that the capturerange of the e, f, g, h focus signal is relatively small, i.e. beforethe e, f, g, h focus signal provides meaningful information, the laserbeam must already be in a close-to-focused condition, compared to thefocus condition required to obtain a meaningful signal from thequadrature detector, which may have a capture range on the order of 100microns or more. However, as noted above, the quadrature detector isrelatively sensitive to optical faults, such as misalignment of opticalcomponents, compared to the optical fault sensitivity of the e, f, g, hfocus signal.

The present invention includes using both the e, f, g, h focus signaland the quadrature detector focus signal. The quadrature detector errorsignal is used to directly control the focus thus maintaining the focussystem sufficiently close to an in-focused condition that the e, f, g, hfocus signal is meaningful (i.e. is within the e, f, g, h capture rangeso as to provide an indication of the true focus condition). When theoptical disk drive is first made operational, the components are alignedand adjusted to initially provide the desired focus of the read/writebeam with respect to the recording medium. As the system is subjected tosuch influences as mechanical shock or vibrations or thermal expansion,optical components become misaligned and optical faults occur as aresult. As noted, the detector of the defocused beam e, f, g, h isrelatively insensitive to such optical faults, while the quadraturedetector is relatively more sensitive to optical faults. Thus, whenoptical faults cause the quadrature detector to erroneously indicate adefocused condition, the true focus condition is nevertheless providedby the e, f, g, h detector, e.g. according to Equation (6) above.According to the present invention, in such a situation, an off-setsignal is combined with the quadrature focus error signal to provide acorrected signal. The offset signal is derived using the e, f, g, hsignal and has a magnitude and sense such that the corrected "S" curveindicates an in-focus condition if, and only if, the e, f, g, h detectorindicates an in-focus condition. The corrected quadrature error signalis relatively insensitive to optical faults because the "S" curve hasbeen offset so as to indicate correct focus whenever the e, f, g, hsignal indicates correct focus.

In summary, the process of providing a corrected error signal accordingto this embodiment of the invention involves the following steps:

(1) Initial setup: quadrature detector signal is calibrated such thatthe "S" curve indicates the true focus condition.

(2) During operation the e, f, g, h focus signal is sampled.

(3) If the e, f, g, h focus signal and the quadrature detector bothindicate an unfocused condition, the focus system is controlled usingthe "S" curve in the usual manner to bring the beam back into thedesired focus condition.

(4) If the e, f g, h focus signal and the quadrature detector provideopposite indications of the focused condition, an offset signal iscombined with the "S" curve to produce a corrected "S" curve.

In this manner, a corrected error focus signal is provided which is bothrelatively insensitive to optical faults and which has a relativelylarge capture range such as about 100 microns or more.

FIG. 10 illustrates another signal detection principle of the presentinvention. In this case, the optical element 136 includes a lens 138 inconjunction with an optical element such as zone plate 140 to image theaperture of the objective lens to the detector plane via the minus firstorder beam. The optical element results in splitting a portion of thebeam reflected from the disk 20 into first and second beams, and alsoimparting astigmatism to at least one of the two beams. The lens 138 canproduce very high contrast interference patterns on the detector 132.Preferably, in the embodiment depicted in FIG. 10, the optical element136 produces little or no intensity of the zeroth order beam, therebyincreasing the total useful intensity of the detected plus first orderand minus first order beams. Construction of an optical element with thedescribed characteristics is described more fully below and inconjunction with Wai-Hon Lee "High Efficiency Multiple Beam Gratings",Applied Optics Vol. 18, No. 13, pp. 2152-2158, incorporated herein byreference. The minus first order beam is a converging beam used forobtaining the astigmatic focus error signal in the manner describedabove in connection with FIG. 8. Both beams as they appear on thedetector 132 are shown in FIG. 10A. A detector 132 identical to the oneshown and described for FIGS. 8 and 8B, can be used in connection withthe arrangement depicted in FIG. 10.

In both the first and second embodiments, the splitting of the reflectedbeam into first and second beams is preferably accomplished usingelliptical (including partially elliptical) zone plates. Such anelliptical zone plate can be made by a photographic method, describedbelow, involving photoreduction and photoresist replication of anelliptical pattern. The pattern can be initially generated by, forexample, a computer which drives a pen plotter, photoplotter, electronbeam or CRT display or other plotting device. The characteristics of thezones which are to be plotted can be obtained by solving one or moreequations which are related to the desired characteristics of theelliptical zone plate.

In relation to both embodiments of the present invention, it is wellknown in optics that a plate consisting of alternating opaque andtransparent concentric circular zones can focus a collimated beam oflight to a point. This plate is sometimes called a Fresnel zone plate.It can also be interpreted as a hologram of a spherical wave interferingwith an online collimated beam. The zone plate of the present inventioncan be understood with the same general principle.

FIG. 11 schematically depicts a spherical wave at a distance F from apoint source 0 which can be represented mathematically as follows:

    φ (r)=(2.sup.π /.sup.λ) [(r.sup.2 +F.sup.2).sup.178 -F], (7)

where (x,y) is the phase variation of the wavefront of the sphericalwave at plane P, .sup.λ is the wavelength of the light and r is theradial distance from the origin Q. The radius R_(n), such that φ (R_(n))is equal to 2n.sup.π, is given by:

    R.sub.n =(n.sup.2λ2 +2n.sup.λ F).sup.178 .   (8)

For large F, as in most practical cases, R_(n) can be represented by:

    R.sub.n =(2n.sup.λ F).sup.1/2.                      (9)

The zone plate with focal lengths F_(x) and F_(y) is obtained byplotting ellipses with major and minor axes given by:

    R.sub.xn =(2nλ F.sub.x).sup.1/2.                    (10)

    R.sub.yn =(2nλ F.sub.y).sup.1/2.                    (11)

The difference in radii of curvature along the two axes producesastigmatism similar to that produced by a conventional toric lens.

In connection with the first embodiment of this invention, thecomputer-generated astigmatic element is a combination of a positivecylinder along x direction and a negative cylinder in the y direction.Mathematically the phase of such an element can be written as

    phase=.sub.90 (x.sup.2 -y.sup.2)/λf'                (12)

where f' is the focal length of the astigmatic element. When thiselement is used in connection with a spherical lens with focal lengthF", the beam along x will focus at f_(x) which is determined by

    f.sub.x.sup.-1 =f'.sup.-1 +F".sup.-1                       (13)

and the beam along y is focused at f_(y) given by

    f.sub.y.sup.-1 =-f'.sup.-1 +F".sup.-1                      (14)

Computer-generated optical elements create many diffracted orders. The+1 order will have the phase given above and the -1 order will have itsconjugate namely

    conjugate phase=-.sup.π (x.sup.2 -y.sup.2)/.sup.λ f' (15)

These two diffracted orders produce the pattern for the focus errorsignals as shown in FIG. 6.

The focal length f' of the astigmatic element is determined from thefocus range required by the system. Suppose that F" is equal to 40 mm.In order to have 3 mm separation between the two astigmatic foci thequations above show that f' is about 1 meter.

To obtain the astigmatic pattern in FIG. 6 we must rotate the astigmaticelement by 45° . The phase in such a case is given by

    phase=2πxy/λf'                                   (16)

To make such an element it is necessary to determine the interference ofsuch a wavefront with a slight tilted collimated beam. The positions ofthe fringes are located by solving the following equation

    2πd/λF"+2πxy/λf'=2πn                (17)

where d is the separation of the first order from the zero order.

In generating a zone plate usable in connection with the secondembodiment of the invention, for the purpose of illustration, it isassumed that in FIG. 10 the distance of the readout objective lens is ata distance D' from the lens 138 which has a focal length F'. The zoneplate 140 has an average focal length of f.

It is also assumed that the detector 132 is located at a distance D fromthe lens 138. The relationship among all of these variables is given bythe following equations:

    f.sup.-1 +F'.sup.-1 =2 D.sup.-1,                           (18)

    -f.sup.-1 =F'.sup.-1 =D.sup.-1.                            (19)

It is possible to determine the focal lengths F' and f from theinformation on D. This is by no means the only relationship that existsbetween F' and f. The astigmatic focal lines in the elliptical zoneplate 140 are mainly determined by the range of the focus errors. Thedistance needed between the two astigmatic focal lines (AD) isapproximately given by:

    AD=2 K (f/g).sup.2,                                        (20)

where K is the focus control range, g is the focal length of theobjective lens and f is the average focal length of the zone plate. Asan example, for K=30 micrometers, g=4 mm and f=30 mm, the AD iscalculated to be about 3.4 mm.

An advantage of the present invention is the simplicity of fabricationof an elliptical zone plate element 130. The manufacture of this elementis similar to the process required in the fabrication of microelectroniccomponents.

FIG. 12 depicts the sequence of steps involved in alternate methods forproducing the phase relief structure for a zone plate element. A glasssubstrate 150 is first coated with a light sensitive material 152 suchas a photoresist or photopolymer. A mask 154 containing thecomputer-plotted and usually photographically reduced image of the zoneplate pattern that is to appear on the zone plate element 130 or 140 isdisposed over the coating material 152. A source of light, such asultraviolet light, is used to expose the light sensitive material 152through the mask 154 (FIG. 12A). After exposure and a proper developmentprocess for the photosensitive material 152, a structure is derived suchas shown in FIG. 12B. This structure can then be subjected to an etchingprocess (such as with hydrofluoric acid or by plasma etching withappropriate gases) to arrive at a surface profile as shown in FIG. 12C.As indicated above, it is preferable that the zone plate remaintransparent, as shown, but the pattern can also be made opaque in anydesired manner.

An alternative method to fabricate the phase structure is to deposit alayer 156 of transparent silicon monoxide (SiO), silicon dioxide (SiO ),or magnesium fluoride (MgF ) on the structure of FIG. 12B to arrive atthat shown in FIG. 12D. Removing the photoresist layer from thestructure shown in FIG. 12D will produce the phase structure shown inFIG. 12F. As long as the thickness of the relief is correct according tothe teaching contained in an article entitled, "High Efficiency MultipleBeam Gratings", by W. H. Lee, Applied Optics. Volume 18 pp. 2152-2158,1979, the structures shown in FIGS. 12E and 12F will have identicalproperties insofar as the ability of such to serve as the zone platestructure described hereinabove. The zeroth order can be suppressed bymaking the layer which corresponds to the computer generated pattern 154to have a thickness 158 about the wavelength of the laser light source.

The manner of using the present invention will now be described.According to the first embodiment of the invention, depicted in FIG. 4,the beam reflected from the data disk 66 is passed through the opticalelement 60 described above to produce first and second beamssubstantially focused on first and second detectors 74, 76. The firstand second detectors produce signals k, l, m, n, p, q, r and s relatedto the amount of light which falls on each quadrant of the respectivequadrature detectors. These signals are combined according to Equations3 and 4 to provide focus and tracking signals respectively which areused to control the tracking and focus of the read/write beam asdescribed above in connection with the first embodiment.

According to the second embodiment depicted in FIGS. 8 and 10, a portionof a laser beam reflected from a data disk 20 is passed through theoptical element 130, 136, as described above, to produce patterns oflight on an optical detector 132. Signals a, b, c, d, e, f, g and h fromthe optical detector, as depicted in FIGS. 8B and 10A, are combined toproduce a tracking error signal and a focus error signal. The trackingerror signal is used to move a tracking device such as a controllablecarrier mirror or arm so as to position the focused spot of theread/write beam onto a desired position of the data disk 20. The focuserror signal is used to position the objective lens 18 to a positionwhich results in providing a desired degree of focus of the read/writebeam on the data plane of the disk 20.

In all aspects of the present invention, signals can be combined orcompensated whether by way of addition, subtraction, multiplication orother combinations, using well-known electronic circuits, such as addingcircuits, difference circuits or the like, or can be combined usingcomputers or microprocessors.

Although the present invention has been described with reference tocertain embodiments, it should be appreciated that further modificationscan be effected within the spirit and scope of the invention.

What is claimed is:
 1. Apparatus useful in maintaining focus in a diskdata-storage system, comprising:radiation source means for producing aread/write beam of radiation; focus means for providing a desired degreeof focus of said read/write beam on the disk, wherein a portion of saidread/write beam is reflected from the disk to provide a reflected beam;optical element means for simultaneously producing from at least aportion of said reflected beam, first and second beams; said first beamhaving an optical axis and being astigmatically focused by said opticalelement means to an image in an image plane; said second beam beingdefocused at said image plane; first detector means substantially insaid image plane providing a first signal varying in response to theshape of said image; second detector means substantially in said imageplane located substantially outside said image and producing a secondsignal in response to said second beam related to said degree of focus;means for combining said first signal and said second signal to producea third signal, said third signal being related to said degree of focusand being substantially constant in response to movement of said opticalaxis; and first control means for changing said degree of focus,responsive to said third signal.
 2. Apparatus, as claimed in claim 1,wherein:said second detector means produces a fourth signal related toradial tracking of said read/write beam.
 3. Apparatus, as claimed inclaim 1, wherein:said second signal is an offset signal having amagnitude such that the sum of said first and second signals produces an"S" curve which indicates an infocus condition when said second detectormeans indicates an in-focus condition; and said means for combiningcomprises an adder circuit for adding said first and second signals. 4.Apparatus, as claimed in claim 1, wherein:said first monitor beam andsaid second beam are coaxial.
 5. Apparatus, as claimed in claim 1,wherein:said optical element means comprises an elliptical zone plate.6. Apparatus, as claimed in claim 1, wherein:said first and seconddetector means form a single compact substantially planar detectionapparatus comprising first and second pairs of substantially adjacentoptical detectors, and a quadrature detector, said quadrature detectorlying substantially midway between said first pair and said second pairof optical detectors.
 7. Apparatus useful in monitoring focus andtracking of a disk data-storage system, comprising:radiation sourcemeans for producing a read/write beam of radiation; focus means forfocusing said read/write beam on the disk wherein a portion of saidread/write beam is reflected from the disk to produce a reflected beam;optical element means for simultaneously producing from said reflectedbeam a first focus control beam and a second tracking control beam, saidfirst focus control beam being astigmatically focused by said opticalelement means to an image in an image plane, and said second trackingcontrol beam being defocused at said image plane, said first and secondbeams being coaxial; first detector means substantially at said imageplane for producing a first signal related to said first focus controlbeam; second detector means to produce a signal related to said secondtracking control beam; first control means for changing the focal planeof said focus means using said first signal; and second control meansfor changing the radial tracking of said read/write beam relative tosaid disk using said second signal.
 8. Apparatus, as claimed in claim 7,wherein:said optical element means comprises an elliptical zone plate.9. Apparatus, as claimed in claim 7, wherein:said second detector meansproduces a third signal related to said degree of focus, said thirdsignal being substantially constant in response to movement of theoptical axis of said reflected beam; and said first control means usessaid third signal for changing the focal plane of said focus means. 10.Apparatus, as claimed in claim 7, wherein:said first detector meanscomprises four optical detectors; said first signal is obtained by usingmeans for subtracting the sum of the outputs from two of said fourdetectors from the sum of the outputs from the remaining two of saidfour detectors.
 11. Apparatus, as claimed in claim 7, whereinsaid firstand said second detector means form a single compact substantiallyplanar detection apparatus comprising first and second pairs ofsubstantially adjacent optical detectors and a quadrature detector lyingsubstantially midway between said first pair of optical detectors andsaid second pair of optical detectors.